1. Field of the Invention
The present invention relates to an optical cavity for an Non-Dispersive Infrared (NDIR) gas sensor. More specifically, the sensors based on the principal of Non-Dispersive Infrared Detection uses the light-absorbing characteristic of gases to measure the amount of light absorption that occur at the specific wavelength that is absorbed by a target gas and calculate the target gas concentration.
2. Related Prior Art
An NDIR gas sensor normally comprises a gas chamber consisting of optical cavity, light source and light detector, and electronic circuits that analyze the electric signal from the light detector to get gas concentration.
In general, in order to obtain an NDIR gas sensor of superior quality with higher precision and accuracy, it is important to extend the length of the optical path within a given optical cavity so that the amount of absorbed light during the propagation of the light within the optical cavity increases.
However, simply increasing the optical cavity to get extended optical path may reduce the competitiveness of a product by causing an increase in the volume of the gas sensor and related manufacturing cost. One effective way of extending the optical path of an optical cavity of a limited size is to efficiently arrange the geometric position of mirrors and lenses in the optical cavity.
Due to the above-mentioned reason, it has been the most common way of designing and analyzing the conventional optical cavity to find the optimal geometric arrangement of the mirrors and lenses through trial and error using optical simulation technique. However, in the optical simulation, too many factors may affect the performance of the optical cavity and the accumulation of a number of small error factors occurring in the designing and manufacturing process of an optical cavity may result in a great deal of deviation in the performance of each product as well as a lot of time and cost consumption for the recovery of errors. Further, in designing an optical cavity using the optical simulation technique, one has to redesign the optical cavity by trial and error process in order to increase or decrease the length of the optical path with a given sized optical cavity and may obtain an impractical optical cavity that cannot be applied to a given gas sensor circuit.
In addition, in order to design an optical cavity of a high efficiency with a long optical path and a small cavity volume, the light radiated from a light source should be detected by a detector after sufficiently circulating inside the optical cavity to make the most use of the inner space of the given optical cavity. Under this situation, however, the space for gas ventilating opening shall be relatively reduced to cause the response time (i.e. time period required for one time measurement) to be excessively extended. That is, it might take several minutes to measure the gas concentration one time and thus the conventional gas cavity with a small space for gas ventilating opening could not be used in a sensing environment requiring a rapid measuring speed (or a short response time).
Still further, when measuring a gas having a good absorption characteristic to a thermal infrared using the NDIR detection (i.e. when measuring CO2 gas that absorbs 4.2 micrometer infrared) the thermal infrared causes thermal vibration of the cavity material and thereby increases the temperature of the optical cavity. For example, when a light beam having energy of 100 is radiated from the light source a greater energy of more than 100 can be formed due to the temperature rise in the optical cavity, which results in the measurement error. In order to minimize this measurement error one may use pulse type light beam. However, efficient gas measurement may not be possible due to the small intensity of radiation at the light detector with a pulse type light beam of a small pulse width. In contrast, the use of a pulse type light beam of a large pulse width may affect the temperature rise inside the optical cavity and thus cause to incur the measurement error of the gas sensor.
Still further, since the characteristic of optical cavity including the length of optical path may be affected by various factors it is not easy to modify the design variables by repeated experiment after designing an optical cavity or to predict the characteristic change of the optical cavity and reflect the change onto the cavity design.
Since the light beam radiated from the light source has a predetermined beam width or beam size it is not possible to make all the radiated light to have an ideal optical path passing through a light focus. Therefore, it is very hard to design an optical cavity in consideration of the various optical paths that arise inside the optical cavity.
The above-mentioned problem prohibits the practical use of the NDIR gas sensor while it has a number of advantages compared to conventional gas sensors of other types.